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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-properties"></a>Interface Properties</h2></div></div></div>
<p>
      Starting from version 2.4 of GLib, GObject interfaces can also have
      properties. Declaration of the interface properties is similar to
      declaring the properties of ordinary GObject types as explained in
      <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>, 
      except that <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-interface-install-property" title="g_object_interface_install_property ()">g_object_interface_install_property</a></code> is used to 
      declare the properties instead of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-install-property" title="g_object_class_install_property ()">g_object_class_install_property</a></code>.
    </p>
<p>
      To include a property named 'name' of type <span class="type">string</span> in the 
      <span class="type">maman_ibaz</span> interface example code above, we only need to
      add one 
      <sup>[<a name="id460708" href="#ftn.id460708" class="footnote">12</a>]</sup>  
      line in the <code class="function">maman_ibaz_base_init</code>
      <sup>[<a name="id460721" href="#ftn.id460721" class="footnote">13</a>]</sup>
      as shown below:
</p>
<pre class="programlisting">
static void
maman_ibaz_base_init (gpointer g_iface)
{
  static gboolean is_initialized = FALSE;

  if (!is_initialized)
    {
      g_object_interface_install_property (g_iface,
                                           g_param_spec_string ("name",
                                                                "Name",
                                                                "Name of the MamanIbaz",
                                                                "maman",
                                                                G_PARAM_READWRITE));
      is_initialized = TRUE;
    }
}
</pre>
<p>
    </p>
<p>
      One point worth noting is that the declared property wasn't assigned an 
      integer ID. The reason being that integer IDs of properties are used
      only inside the get and set methods and since interfaces do not
      implement properties, there is no need to assign integer IDs to
      interface properties.
    </p>
<p>
      An implementation shall declare and define it's properties in the usual
      way as explained in <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>, except for one
      small change: it must declare the properties of the interface it
      implements using <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-override-property" title="g_object_class_override_property ()">g_object_class_override_property</a></code>
      instead of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-install-property" title="g_object_class_install_property ()">g_object_class_install_property</a></code>.
      The following code snippet shows the modifications needed in the
      <span class="type">MamanBaz</span> declaration and implementation above:
</p>
<pre class="programlisting">

struct _MamanBaz
{
  GObject parent_instance;

  gint instance_member;
  gchar *name;
};

enum
{
  PROP_0,

  PROP_NAME
};

static void
maman_baz_set_property (GObject      *object,
                        guint         property_id,
                        const GValue *value,
                        GParamSpec   *pspec)
{
  MamanBaz *baz = MAMAN_BAZ (object);
  GObject *obj;

  switch (prop_id)
    {
    case ARG_NAME:
      g_free (baz-&gt;name);
      baz-&gt;name = g_value_dup_string (value);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
maman_baz_get_property (GObject    *object,
                        guint       prop_id,
                        GValue     *value,
                        GParamSpec *pspec)
{
  MamanBaz *baz = MAMAN_BAZ (object);

  switch (prop_id)
    {
    case ARG_NAME:
      g_value_set_string (value, baz-&gt;name);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
maman_baz_class_init (MamanBazClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  gobject_class-&gt;set_property = maman_baz_set_property;
  gobject_class-&gt;get_property = maman_baz_get_property;

  g_object_class_override_property (gobject_class, PROP_NAME, "name");
}

</pre>
<p>
    </p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.id460708" href="#id460708" class="para">12</a>] </sup>
          That really is one line extended to six for the sake of clarity
        </p></div>
<div class="footnote"><p><sup>[<a id="ftn.id460721" href="#id460721" class="para">13</a>] </sup>
          The <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-interface-install-property" title="g_object_interface_install_property ()">g_object_interface_install_property</a></code>
          can also be called from <code class="function">class_init</code> but it must
          not be called after that point.
        </p></div>
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